Today, television is in the midst of a transformation. Standard television receivers capable of receiving and presenting signals broadcast since the 1940s are being replaced by televisions capable of presenting higher definition signals (so called high definition (HD) signals). HD signals are typically broadcast as digital signals, and currently result in images having resolutions of up to 1920×1080 pixels. This is a vast improvement over conventional standard definition (SD) signals that typically result in images having between 480 and 525 lines, each equivalent to between about 640 to 720 pixels. HD signals thus allow for images having greater image detail and sharpness, allowing for an enhanced viewing experience. As an added benefit, television images may be presented on larger and larger displays.
As many existing television broadcasts are still in SD format, and as many existing programs have been stored and recorded in SD format, newer HD televisions are typically capable of presenting both HD and SD images. SD images are simply enlarged or scaled to fit available space on an HD display.
However, any time an SD image is scaled to HD, a relatively low content signal is converted for presentation into a display format that has more pixels and allows for more rapid image transitions. The scaled SD image is thus far less sharp than a true HD image.
To address deficiencies in scaled SD signals, edge enhancers are often used to enhance edges within the up-scaled image. Good edge enhancement is difficult to achieve, because it simultaneously requires actively speeding up slower edges without causing ringing. Ringing arises because edge-enhancing adds high-frequency components to signals which tend to ring when scaled.
Known edge enhancers add the incoming signal to a differentiated, amplified, and clipped version of this signal. Such enhancers were initially designed to enhance VCR images with frequency responses well below that of broadcast television. Early edge enhancers operated only on the colour (or UV) portion of the signal. The techniques have since been extended to the luminance portion of the signal.
Differentiation, however, inherently emphasizes high frequency noise. Thus, these conventional edge enhancers basically enhance the low frequencies that are already there—they do not really modify the edge and add new high frequencies, except possibly by non-linear clipping. This leads to the “edge around the object” type of sharpness enhancement that is commonly observed on existing TVs.
Further, these edge enhancers often require a circuit which must explicitly decide when an edge is present using a logic circuit. Such decision circuits often fail on complicated source images. Methods which alter the time scale to move the regions before or after an edge are typical of those which require this type of decision. Further, the decision circuits are confused by thin or multiple or close edges. To alleviate this problem, complex and often ineffective circuits may alter how edges are processed in regions of close multiple edges.
Accordingly, there is a need for new edge enhancement techniques and circuits.